Turbulence in sheared, salt-fingering favorable environment

Abstract:

Instability and turbulence in sheared, salt-fingering favorable stratification are studied
using three-dimensional direct numerical simulations (DNS). Salt-fingering favorable
stratification is gravitationally stable, because the unstable vertical gradient of salinity
is stabilized by temperature (warm, salty over cool, fresh water-masses). Salt-fingering
instability can occur at the interface of these different water-masses. Salt-fingering instability
generates cells of rising and sinking fluid because of the difference in diffusivity
of heat and salt. In the presence of a vertically varying horizontal current (shear), saltfingering
instability is supplanted by salt-sheet instability. Salt-sheet instability generates
alternating planar regions of rising and sinking fluid, aligned parallel to the direction of
the sheared current.
As the salt sheet reaches the finite amplitude, secondary instability appears at the
edges of salt sheets and introduces quasi-periodic dependence along the direction of the
sheared current. The secondary instability disrupts the growth of salt sheets and brings
the flow into the turbulent regime. Secondary instability can be treated approximately as
linear normal mode of the finite-amplitude salt sheets. The secondary instability is shown
to be an oscillatory instability, driven primarily by buoyancy.
In the turbulent regime, it is shown that thermal and saline buoyancy gradients become more isotropic than the velocity gradients in the dissipation-range scale. In the
velocity field, the geometry of the primary instability is embedded in the dissipation range
scale geometry even in the turbulent regime; therefore, the flow geometry from
primary instability biases the estimation of the turbulent kinetic energy dissipation rate.
Estimation of the turbulent kinetic energy dissipation rate by assuming isotropy, a common
method in the interpretations of observations, can underestimate its true value by a factor
of 2 to 3.
Of primary interest of the oceanographic community is the turbulent transport
of momentum, heat, and salt associated with salt-sheet instability, which can modify
water-masses and lower the potential energy of the ocean. The effective diffusivites of
momentum, heat, and salt are used to describe the turbulent state. The effective diffusivity
of momentum is an order of magnitude smaller than that of salt; turbulence associated with
salt-sheet instability is therefore relatively inefficient in transferring momentum. These
effective diffusivities are compared to observational estimates.